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Eucryptite Filler for CTE Control of Organic Substrate

Keywords: PCB, CTE, Organic Substrate

Current printed circuits boards (PCBs) are all organic, the most common being epoxy-glass laminate FR-4 due to its low cost and overall performance. However, for high-density interconnection(HDI) and assembly of flip-chips directly to the substrate and 3-dimensional packaging(PoP etc), substrate materials with low coefficient of thermal expansion(CTE) are great needed. A limiting problem in electronic packaging is thermally induced stresses in Integrated circuits die and substrates because of different CTE.
Composite technology can be utilized to minimize CTE of substrate. An important method for CTE control is the use of fillers with high volume fractions. By increasing volume fraction of filler, it is possible to reduce CTE of substrate and solder mask materials. From above attempts, CTE of FR-4 can! be reduced to 10ppm/?. However, There are some disadvantages, for example, high cost, hard to process, and failure of reliability because of low adhesion with Cu. Furthermore, 10ppm/? is not sufficient to solve problem of HDI and warpage of package substrate.
In our studies to solve these problems, we use negative CTE materials as filler. This is Eucryptite. We investigated CTE of eucryptite as a function of sintering temperature and time. The CTE of eucryptite sintered at 1300? is -9ppm/?. It is very lower value than that of fused silica(=0.5ppm/?) used as filler of c! onventional FR-4. The CTE of epoxy and 30 weight percent of eucryptite is 30ppm/?. We investigated their mechanical, electrical properties. Also, reliability test are conducted.